As wells are drilled to greater lengths and depths, it becomes necessary to provide a liner (usually casing or some other tubing string) to avoid undesirable fluid inflows or outflows and to prevent borehole collapse. The annular space between the borehole wall and the liner is usually filled with cement to reinforce structural integrity and to prevent fluid flows along the outside of the liner. If such fluid flows are not prevented, there is a loss of zonal isolation. Fluids from high-pressured formations can enter the borehole and travel along the outside of the liner to invade lower-pressured formations, or possibly to exit the borehole in a mixture that dilutes the desired production fluid. Results may include contamination of aquifers, damage to the hydrocarbon reservoir, and loss of well profitability.
The job of cementing the liner in place has several potential pitfalls. For example, as the borehole wall can be quite irregular, the volume of the annular space between the liner and the borehole wall is somewhat unpredictable. Moreover, there may be voids, fractures, and/or porous formations that allow cement slurry to escape from the borehole. Conversely, fluids (including gasses) can become trapped and unable to quickly escape from the annular space, thereby preventing the cement slurry from fully displacing such materials from the annular space. (Any such undisplaced fluids provide potential paths for fluid flow that can lead to a loss of zonal isolation.) Accordingly, the cementing crew may have difficulty predicting how much of the well will be successfully cemented by a given volume of cement slurry. Inaccurate estimates may lead to the use of too much or too little cement slurry and improper placement, any of which can reduce the utility and profitability of the well.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed with the given embodiments by the scope of the appended claims.